Method and a device for making images of the quantum efficiency of the photosynthetic system with the purpose of determining the quality of plant material and a method and a device for measuring, classifying and sorting plant material

ABSTRACT

The present invention relates to a method for determining the quality of plant material by making chlorophyll fluorescence images of said material by scanning the material with a beam of electromagnetic radiation so that the chlorophyll present is excitated, and measuring the chlorophyll fluorescence with an imaging detector. From the fluorescence images obtained with a fast and a slow scan, the image of the quantum efficiency of the photosynthetic system of the plant material is calculated. The invention further relates to a device for measuring the chlorophyll fluorescence images and to a method and devices for sorting and classifying plant material.

The present invention relates to a method for determining the quality ofplant material, such as for instance whole plants, leaf material,fruits, berries, flowers, flower organs, roots, seeds, bulbs, algae,mosses and tubers of plants, by making chlorophyll fluorescence images,particularly a method wherein a characteristic chlorophyll fluorescenceimage is calculated from the measured chlorophyll fluorescence imagesand more particularly a method wherein said characteristic fluorescenceimage contains information about the quantum efficiency of thephotosynthetic activity of photosynthetic system of the plant material.The present invention further relates to a device for measuring thechlorophyll fluorescence images and calculating the image of the quantumefficiency of the photosynthetic activity of the photosynthetic systemof the plant material from said chlorophyll fluorescence images. Thepresent invention also relates to a device for sorting and classifyingplant material on the basis of the chlorophyll fluorescence images andthe image of the quantum efficiency of the photosynthetic activity ofthe photosynthetic system of the plant material calculated from saidchlorophyll fluorescence images.

PRIOR ART

The common method of measuring the quantum efficiency of thephotosynthetic activity of plant material, is measuring thephotosynthetic activity using U. Schreiber's pulse amplitude modulation(PAM) fluorometer described in “Detection of rapid induction kineticswith a new type of high frequency modulated chlorophyll fluorometer”Photosynthesis Research (1986) 9: 261-272. In this method the quantumefficiency of the photosynthetic activity is determined. To that endfirst the fluorescence yield, FO, is measured in the dark or at a lowlight intensity of the ambient light. Then the maximum fluorescenceyield, Fm, is determined at a saturating light pulse. From the twomeasuring signals the efficiency of the photosynthetic system can becalculated according to Q=(Fm−FO)/Fm. Said measuring method determinesthe efficiency of the photosynthetic system of a small surface of aleaf, a so-called spot measurement and therefore is not imaging.

Known measuring methods that are imaging, work according to the sameprinciple as the PAM fluorometer. A known measuring method is the one ofB. Genty and S. Meyer, described in “Quantitative mapping of leafphotosynthesis using chlorophyll fluorescence imaging” AustralianJournal of Plant Physiology (1995) 22: 277-284. In this method thesurface of the plant material, for instance a leaf, is irradiated inshort pulses with electromagnetic radiation from a lamp and thefluorescence is measured during the pulses with a camera system. Saidfirst measurement takes place in the dark or at a low light intensityand results in the FO measurement. The next measurement is carried outat a saturating light pulse and results in the Fm measurement. From saidmeasurements an image can be calculated of the efficiency of thephotosynthetic system. A drawback of this method is that a large surfaceof for instance 50×50 cm² cannot be irradiated with a saturating lightpulse. The present light sources are not bright enough to irradiate sucha surface with sufficient light intensity.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method formeasuring the chlorophyll fluorescence in an imaging manner, and todetermine the quantum efficiency of the photosynthetic activity of plantmaterial from the obtained chlorophyll fluorescence images, wherein thedrawback of the small measuring surface of the known measuring methodsis overcome.

The present invention therefore provides a method for determining thequality of plant material by determining a chlorophyll fluorescenceimage of said plant material, wherein the plant material is irradiatedwith a beam of electromagnetic radiation comprising one or more suchwavelengths that at least a part of the chlorophyll present is excitatedby at least a part of the radiation, the beam of electromagneticradiation having such a shape that only a small part of the plantmaterial is irradiated, and the beam being moved over the plant materialsuch that a larger part of the plant material is measured, wherein thefluorescence radiation originating from the plant material associatedwith the chlorophyll transition, is measured with an imaging detectorfor obtaining a chlorophyll fluorescence image.

Preferred is such a method, wherein, in any given order, during acertain duration of time several fast scans are made over the plantmaterial with the electromagnetic beam for obtaining a chlorophyllfluorescence image Ffast, and during a certain duration of time a slowscan is made over the plant material with the electromagnetic beam forobtaining a chlorophyll fluorescence image Fslow, and subsequently thecharacteristic chlorophyll fluorescence image that is a measure for theefficiency of the photosynthetic system of plant material is calculatedfrom the chlorophyll fluorescence images. Ffast and Fslow.

Preferably the characteristic chlorophyll fluorescence image containsinformation about the quantum efficiency of the photosynthetic activityof the photosynthetic system of the plant material and this image iscalculated with the formula IQP=(Fslow−Ffast)/Fslow.

SHORT DESCRIPTION OF THE FIGURES

FIG. 1 schematically shows a device for making chlorophyll fluorescenceimages and determining the quantum efficiency of the photosyntheticactivity of plant material.

FIG. 2 shows three chlorophyll fluorescence images that have beenobtained using a device according to FIG. 1 for black nightshade. PanelA shows the result of a number of fast scans; panel B shows the resultsof a slow scan; panel C shows the result of the quantum efficiency ofthe photosynthetic activity, calculated from the images of panel A andB.

DETAILED DESCRIPTION

The present invention is based on a spectroscopic measurement which ishighly specific to the chlorophyll present and the functioning of thephotosynthetic system. The functioning of the photosynthetic system isvery important to the proper functioning of a plant and the quality ofthe plant. Light is captured by the chlorophyll molecules. If the plantis of a good quality and is not subjected to stress, the captured energyof the chlorophyll molecules will quickly be passed on to thephotosynthetic system for conversion into chemical energy. Chlorophyllhas the property that it shows fluorescence. When the energy can beprocessed sufficiently fast by the photosynthetic system this results ina low level of fluorescence light. When the photosynthetic system cannotprocess the energy sufficiently fast, the fluorescence light willincrease in intensity. When upon switching on a saturating light sourcehaving electromagnetic radiation which is absorbed by the chlorophyll,the photosynthetic system is able to process the energy fast, theduration of time from switching on the light source until the maximumlevel of the irradiated fluorescence will be much longer, than would bethe case when the photosynthetic system is not able to process theenergy fast. This property is now utilised to determine the quantumefficiency of the photosynthetic activity. The method of the inventionmakes it possible to measure the quantum efficiency of thephotosynthetic activity of whole plants in an imaging manner.

In the method of the invention plant material is irradiated withelectromagnetic radiation having such a wavelength that at least a partof the chlorophyll present is excitated, for instance withelectromagnetic radiation having a wavelength of between 200 and 750 nm,such as laser light having a wavelength of approximately 670 nm. Thefluorescence is measured with an imaging detector, for instance with acamera, between 600 and 800 nm, for instance around 730 nm. The beam ofelectromagnetic radiation may for instance be obtained by using a laserproducing a diverging laser beam which is scanned over the plantmaterial by means of a moving mirror, for instance a rotatable mirrormounted on a galvanometer and controlled by a computer. The plantmaterial can now first be scanned fast with the laser line with afrequency of between approximately 1 Hz and approximately 10 kHz, forinstance with a frequency of 50 Hz, during for instance 10 seconds.During those 10 seconds the fluorescence is measured by an imagingdetector. This image is called the Ffast measurement which istransmitted to the computer. Subsequently a slow scan can be made with afrequency of between approximately 0.01 and approximately 1 Hz, forinstance a frequency of 0.1 Hz during a same duration of time of 10seconds. During said 10 seconds the fluorescence is measured again by animaging detector. Said image is called the Fslow measurement and it isalso transmitted to the computer. From these two images the quantumefficiency of the photosynthetic activity (Imaging Quantum efficiency ofPhotosynthesis: IQP) can be calculated according to:IQP=(Fslow−Ffast)/Fslow   (1)

The computer carries out the calculation according to formula (1) foreach image pixel of the plant material. This results in thecharacteristic chlorophyll fluorescence image as an intensitydistribution of the quantum efficiency of the photosynthetic system ofthe plant material. If during the measurement the duration of time ofthe fast scan is riot the same as the duration of time of the slow scanthe calculation has to be corrected therefore.

For irradiating the plant material a laser, lamp or LED lamp can be usedwhich irradiates the plant material with electromagnetic radiation inthe shape of a thin line or another shape, such that during a scan withthe electromagnetic radiation over the plant material a small part ofthe plant material is irradiated and in a scan by moving the mirror alarger part of the plant material or the entire plant material isirradiated during a certain duration of time. Any movable or rotatablemirror can be used as the mirror, such that the electromagneticradiation is reflected by the mirror and scanned over the plantmaterial. An electrically controlled galvanometer, a movable mirror onspring steel, a polygon mirror or another known structure can be usedfor moving the mirror. The fluorescence radiation originating from theplant material can be measured with any suitable imaging detector, forinstance a video camera, CCD-camera, line scan camera or a number ofphotodiodes or photomultipliers.

The intensity and width and length of the electromagnetic radiation, orthe power of the electromagnetic radiation per surface unit, which isscanned over the plant material, are preferably selected such that thephotosynthetic system during a slow scan is saturated. The frequenciesassociated with a slow and fast scan are selected such that the valuecalculated for the quantum efficiency of the photosynthetic activityaccording to formula (1) corresponds within certain limits with ameasurement of Schreiber's PAM-fluorometer. The time it takes to make aslow scan can be taken as the duration of time of measuring a fast andslow scan.

The invention is highly sensitive, entirely non-destructive and imaging.These are the characteristics of the invention that enable one to make asorting device or classifying device by which means plant material canbe selected or classified on the basis of the IQP measurement. Becausethe IQP measurement has a direct relation to the quality of the plantmaterial, sorting or classification on quality is possible.

The invention therefore also relates to a method for separating orclassifying plant material consisting of individual components intoseveral fractions each having a different quality, wherein acharacteristic image parameter is determined for each component using amethod or device for determining the quality of plant material accordingto the invention and the fractions of components having a characteristicimage parameter in the same pre-determined range are collected.

The invention further relates to a device for separating plant materialusing the above-mentioned method,.comprising a supply part for the plantmaterial, a part for the irradiation, a part for the measuring of thefluorescence radiation originating from the plant material for obtainingthe fluorescence image and image of the quantum efficiency of thephotosynthetic activity and a separation part that works on the basis ofthe image measured.

The invention further relates to a method for classifying plant materialusing the above-mentioned method, comprising a moving structure forlocalising the plant material, for instance a moving carriage or robotarm, a part for the irradiation, a part for the measuring of thefluorescence radiation originating from the plant material for obtaininga fluorescence image of the quantum efficiency of the photosyntheticactivity and a classification part that works on the basis of the imagemeasured.

The material to be sorted or classified may consist of entire plants,cut flowers, leaf material, fruits, berries, vegetables, flowers, flowerorgans, roots, tissue culture, seeds, bulbs, algae, mosses and tubers ofplants etc. The fractions into which the plant material is separated orclassified, may each consist of individual entire plants, cut flowers,leaf material, fruits, berries, vegetables, flowers, flower organs,roots, tissue culture, seeds, bulbs, algae, mosses and tubers of plantsetc.

The present invention can be used for refined purposes, such as earlyselection of seedlings on stress tolerance, programmed administering ofherbicides and quality control in greenhouse culture. The methodaccording to the invention can be used in the screening of the plantquality in the seedling stage at the grower's. Trays of seedlings can betested. Seedlings of a low quality can be removed and replaced by goodseedlings. The method according to the invention can also be used forselection of seedlings on stress sensitivity by subjecting the trays toinfectious pressure or to abiotic stress factors and registering thesignal build-up on-line. In this connection the specific demands thatare made on the quality of seedlings by biological farming, areinteresting. Damage to plant material due to diseases can be detected ata very early stage in the chlorophyll fluorescence image as a localincrease of the fluorescence. This is detected in the IQP image as alocal decrease of the quantum efficiency of the photosynthetic activity.At an auction plants can be checked on quality. A fast, non-destructiveand objective method for determining the pot plant quality and the vasequality of the flowers supplied at the auction or even duringcultivation is of great economic importance. The flower quality dependson the age, cultivation and possible post-harvest treatment thatinfluence the IQP image. The method according to the invention can alsobe used in high-throughput-screening of model crops (Arabidopsis andrice) for functional genomics research for the purpose of functionanalysis and trait identification. Another important use of the newinvention can be found in the determination of freshness of vegetablesand fruits and the presence of damages, for instance in the form ofdiseases. Damages show a lower IQP value in the IQP image than thehealthy parts of the plant material.

In general it has to be established from tests at what IQP value in theimage sorting or classifying can take place. In a test of several stagesof damages, the IQP value in the image of the damage is measured anddivided into several classes. Subsequently during the growth or storageit is established which classes result in a high quality. The thresholdvalue found in this test is used as the value of IQP to select on.

A preferred embodiment of a device for measuring the chlorophyllfluorescence images and calculating the image of the quantum efficiencyof the photosynthetic activity is shown in FIG. 1. This is a simple formthat the device may have. A laser having a wavelength of between 200 and750 nm, and preferably of 670 nm, (1) produces a diverging laser beamwhich is reflected by a mirror (2) in the direction of the plantmaterial (4). The mirror is mounted on a galvanometer and namely suchthat the mirror can rotate. The galvanometer is controlled by a computer(6) such that the laser line (3) which is generated by the laser can bescanned over the plant material. The laser line preferably has a lengthlarger than the maximum width of the plant material. The laser lineserves to excitate chlorophyll molecules. At least a part of thechlorophyll molecules gets into an electronically excitated state. Atleast a part of the chlorophyll molecules falls back to the ground stateunder emitting fluorescence. The fluorescence is measured with a cameraprovided with an optical filter, suitable to transmit only light between600 and 800 nm, for instance about 730 nm. The method now consists offirst scanning a laser line over the object fast, for instance with afrequency of 50 Hz and during 10 seconds. During said 10 seconds thefluorescence is measured by the camera and read by the computer afterthe measurement. This image is called the Ffast measurement.Subsequently a slow scan is made using for instance a frequency of 0.1Hz during the same duration of time of 10 seconds. During said 10seconds the fluorescence is measured by the camera and read by thecomputer after the measurement. This image is called the Fslowmeasurement. From said two images the quantum efficiency of thephotosynthetic system (IQP) is calculated according to formula (1) foreach pixel of the image.

A skilled person will recognise that for obtaining the image of thequantum efficiency of the photosynthetic system the slow scan can alsobe carried out first.

A device for sorting plant material according to the invention mayconsist of a conveyor belt for the supply of plant material to themeasuring part where the above-mentioned fluorescence measurementaccording to the invention is carried out after which the plant materialis transported further to the separation part in which the fractions ofwhich the IQP image is not within the predetermined limits, are removedfrom the conveyor belt in a manner that is known per se, for instance bymeans of an air flow. The air flow may be regulated by a valvecontrolled by an electronic circuit such as a microprocessor processingthe signal of the measuring part. The plant material may also beseparated in various classes of quality in which for each class ofquality the IQP image of the plant material is within predeterminedlimits. The limits may be established by for instance determining theIQP image of samples of plant material having the wanted quality orproperties. The person skilled in this field will know that the plantmaterial to be separated can also be transported through the measuringpart and the separation part in another way than by means of a conveyorbelt and that various methods are available to sort the variousfractions from the main flow, such as an air flow, liquid flow ormechanic valve. The plant material may for instance also be present in aliquid. Sorting in a liquid may for instance take place in order tominimise the risk of damaging very delicate plant material, such asapples, berries and other soft fruit.

It is further noted that a device for sorting or classifying plantmaterial in for instance a greenhouse or in the field, according to theinvention may consist of a device that runs past the plants and measurestheir IQP image and subsequently classifies them on quality and storesthis in a data base or removes the plant material of inferior quality.The object of a data base is to get an insight into the quality of theentire lot and to enable to quickly retrieve the position of the plantsthat fall within a certain class of quality.

The above-mentioned preferred embodiment for the measurement can also bemoved over the plant material by a robot arm or a known device such as acarriage, for the purpose of measuring deviations in the plant material,such as for instance early detection of diseases. Detection of diseasesin for instance plants can be established because in a test it has beenshown that due to the damage the fluorescence signal at the damaged spotis locally higher or the IQP value is lower than in the surroundingplant material. In tests it has also been established what quantity offungicide has to be applied on the damaged spot to combat the disease.The present invention now allows detecting and locally controlling adisease by locally and in a highly dosed manner spraying the damage witha fungicide in an automated manner by using a nozzle. An advantage ofthe method used is the decrease of the quantity of fungicide, so thatplants need not be sprayed with fungicide by way of prevention.

It is also noted that the device can be used for controlling thecultivation of plants by coupling the greenhouse climate control to theinformation obtained with the method as described above. An advantage ofthe present invention is that the entire plant is imaged and thus a goodmeasure for the quantum efficiency of the photosynthetic activity can becalculated, this as opposed to the PAM fluorometer which only measures asmall part of a leaf.

The invention can be used in any sorting device for plants or fruit. Itis possible to build it in into every sorting device and carriages orrobots that may or may not be automatically propelled.

EXAMPLE 1

This example describes the effect of a herbicide treatment on thechlorophyll fluorescence image and the image of the quantum efficiencyof the photosynthetic activity. The fluorescence images were measuredusing the above-mentioned preferred embodiment according to FIG. 1.

FIG. 2A shows the result of the chlorophyll fluorescence image of thefast scan of a black nightshade plant on which 48 hours previously, oneach of a number of leaves a drop of 3 μl of herbicide solution wasapplied. The herbicide activity is visible in the image in the locallighter shade of the leaves. FIG. 2B shows the result of the slow scanof the same plant. The image of the quantum efficiency of thephotosynthetic activity is calculated with a computer for each pixel ofthe image according to formula (1) from the images 2A and 2B. The darkareas in the image of the leaves are hardly photosynthetically active.The pixels have a value of between 0 and 0.3. The healthy parts of theplant indeed show a normal value of the quantum efficiency of thephotosynthetic activity. The pixels have a value of between 0.7 and0.85. They can be recognised by the light areas. From tests it is knownat what threshold values for the quantum efficiency of thephotosynthetic activity the leaves die. Above a certain threshold valueof the quantum efficiency of the photosynthetic activity those plantparts are still healthy. Below a certain threshold value those plantparts die. From this test it appeared that the threshold value wasapproximately 0.5. An advantage of the present invention is that now theentire plant is measured and therefore a proper judgement can be made ofthe total quantum efficiency of the photosynthetic activity of theentire plant. This as opposed to the methods known up until now in whichat a number of spots of the plant a spot measurement is carried out oronly a small part of the plant is imaged.

1-19. (canceled)
 20. A method for determining the quality of plantmaterial by determining a chlorophyll fluorescence image of said plantmaterial, wherein the plant material is irradiated with a beam ofelectromagnetic radiation comprising one or more such wavelengths thatat least a part of the chlorophyll present is excitated by at least apart of the radiation, the beam of electromagnetic radiation having sucha shape that only a small part of the plant material is irradiated, andthe beam being moved over the plant material such that a larger part ofthe plant material is irradiated, wherein the fluorescence radiationoriginating from the plant material associated with the chlorophylltransition, is measured with an imaging detector for obtaining achlorophyll fluorescence image, wherein, in any given order, during acertain duration of time several fast scans are made over the plantmaterial with the electromagnetic beam for obtaining a chlorophyllfluorescence image Ffast, and during a certain duration of time a slowscan is made over the plant material with the electromagnetic beam forobtaining a chlorophyll fluorescence image Fslow, and subsequently thecharacteristic chlorophyll fluorescence image that is a measure for theefficiency of the photosynthetic system of plant material is calculatedfrom the chlorophyll fluorescence images Ffast and Fslow.
 21. A methodaccording to claim 20, the characteristic chlorophyll fluorescence imagecontaining information about the quantum efficiency of thephotosynthetic activity of the photosynthetic system of the plantmaterial and this image being calculated with the formulaIQP=(Fslow−Ffast)/Fslow
 22. A method according to claim 20, the beamhaving the shape of a thin line.
 23. A method according to claim 20, thebeam being moved such over the plant material that the entire surface ofthe plant material is irradiated.
 24. A method according to claim 20,the electromagnetic radiation used for irradiating the plant materialhaving a wavelength of between 200 and 750 nm.
 25. A method according toclaim 20, the electromagnetic radiation used for irradiating the plantmaterial being generated by a lamp, laser of LED-lamp.
 26. A methodaccording to claim 20, the fluorescence radiation originating from theplant material being measured between 600 and 800 nm.
 27. A methodaccording to claim 20, the fluorescence radiation originating from theplant material being measured with an electronic camera consisting of avideo camera, CCD-camera, line scan camera or a number of photodiodes orphotomultipliers.
 28. A method according to claim 20, the characteristicchlorophyll fluorescence image containing information about the quantumefficiency of the photosynthetic activity of the photosynthetic systemof the plant material and this image being calculated with the formulaIQP=(Fslow−Ffast)/Fslow the beam having the shape of a thin line andbeing moved such over the plant material that the entire surface of theplant material is irradiated, said electromagnetic radiation beinggenerated by a lamp, laser of LED-lamp and having a wavelength ofbetween 200 and 750 nm, said fluorescence being measured between 600 and800 nm with an electronic camera consisting of a video camera,CCD-camera, line scan camera or a number of photodiodes orphotomultipliers.
 29. A device for determining the quality of plantmaterial using a method as defined in claim 20, comprising first meansfor irradiating the plant material with a beam of electromagneticradiation comprising one or more such wavelengths that at least a partof the chlorophyll present in the plant material is excitated, firstmeans for scanning the beam of electromagnetic radiation over the plantmaterial with a high scan frequency, first means for measuring thefluorescence radiation originating from the plant material for obtaininga chlorophyll fluorescence image (Ffast) associated with the fast scan,second means for irradiating the plant material with a beam ofelectromagnetic radiation comprising one or more such wavelengths thatat least a part of the chlorophyll present in the plant material isexcitated, second means for scanning the beam of electromagneticradiation over the plant material with a low scan frequency, secondmeans for measuring the fluorescence radiation originating from theplant material for obtaining a chlorophyll fluorescence image (Fslow)associated with the slow scan and means for processing the chlorophyllfluorescence images Ffast and Fslow, wherein said processing means isprovided with calculating means for calculating a characteristicchlorophyll fluorescence image that is a measure for the quantumefficiency of the photosynthetic activity of the photosynthetic systemof the plant material.
 30. A device according to claim 29, the first andsecond means for irradiating the plant material consisting of the samelaser wherein the laser line is scanned with a high frequency and a lowfrequency, respectively, over the plant material, the first and secondmeans for measuring the chlorophyll fluorescence images consisting of acamera connected to a computer and the means for processing thefluorescence images consisting of a computer provided with software forprocessing the chlorophyll fluorescence images of the fast and the slowscan, wherein said software performs the step of calculating acharacteristic chlorophyll fluorescence image that is a measure for thequantum efficiency of the photosynthetic activity of the photosyntheticsystem of the plant material from Ffast and Fslow.
 31. A method forseparating plant material consisting of individual components intoseveral fractions each having a different quality, wherein acharacteristic parameter is determined for each component using themethod as defined in claim 20 and the fractions of components having thecharacteristic parameter in the same pre-determined range are collected.32. A method according to claim 31, the plant material consisting ofplants, cut flowers, leaf material, fruits, berries, vegetables,flowers, flower organs, roots, tissue culture, seeds, bulbs, algae,mosses and tubers of plants.
 33. A method according to claim 32, eachindividual component consisting of separate plants, cut flowers, leafmaterial, fruits, berries, vegetables, flowers, flower organs, roots,tissue culture, seeds, bulbs, algae, mosses and tubers of plants.
 34. Adevice for separating plant material consisting of individual componentsinto several fractions each having a different quality, comprising asupply part for the plant material, a device as defined in claim 29,that determines a characteristic parameter for each component, and aseparation part that separates the components into fractions ofcomponents having the characteristic parameter in the samepre-determined range.
 35. A device for classifying plant materialconsisting of individual components into several fractions each having adifferent quality, comprising a moving structure that localises theplant material, a device as defined in claim 29 that determines acharacteristic parameter for each component, and a classification partthat collects fractions of components having the characteristicparameter in the same pre-determined range.
 36. A device for determiningthe quality of plant material, comprising a light source for irradiatingthe plant material with a beam of electromagnetic radiation comprisingone or more such wavelengths that at least a part of the chlorophyllpresent in the plant material is excitated, a first beam scanner forscanning the beam of electromagnetic radiation several times over theplant material with a high scan frequency, a first detector formeasuring the fluorescence radiation originating from the plant materialfor obtaining a chlorophyll fluorescence image (Ffast) associated withthe fast scan, a second light source for irradiating the plant materialwith a beam of electromagnetic radiation comprising one or more suchwavelengths that at least a part of the chlorophyll present in the plantmaterial is excitated, a second beam scanner for scanning the beam ofelectromagnetic radiation over the plant material with a low scanfrequency, second means for measuring the fluorescence radiationoriginating from the plant material for obtaining a chlorophyllfluorescence image (Fslow) associated with the slow scan and a processorfor processing the chlorophyll fluorescence images Ffast and Fslow,wherein said processor is provided with a calculator for calculating acharacteristic chlorophyll fluorescence image that is a measure for thequantum efficiency of the photosynthetic activity of the photosyntheticsystem of the plant material.
 37. A device according to claim 36, saidfirst and second light sources consisting of the same laser wherein thelaser line is scanned with a high frequency and a low frequency,respectively, over the plant material, the first and second detectorsconsisting of a camera connected to a computer and the processorconsisting of a computer provided with software for processing thechlorophyll fluorescence images of the fast and the slow scan, whereinsaid software performs the step of calculating a characteristicchlorophyll fluorescence image that is a measure for the quantumefficiency of the photosynthetic activity of the photosynthetic systemof the plant material from Ffast and Fslow.
 38. A device for determiningthe quality of plant material comprising a computer that calculates acharacteristic chlorophyll fluorescence image that is a measure for thequantum efficiency of the photosynthetic activity of the photosyntheticsystem of the plant material from chlorophyll fluorescence images Ffastand Fslow, wherein Ffast is obtained by scanning with a high frequency abeam of electromagnetic radiation comprising one or more suchwavelengths that at least a part of the chlorophyll present is excitatedby at least a part of the radiation, and having such a shape that only asmall part of the plant material is irradiated, over the plant materialand measuring the fluorescence radiation originating from the plantmaterial associated with the chlorophyll transition and Fslow isobtained by scanning with a low frequency a beam of electromagneticradiation comprising one or more such wavelengths that at least a partof the chlorophyll present is excitated by at least a part of theradiation, and having such a shape that only a small part of the plantmaterial is irradiated, over the plant material and measuring thefluorescence radiation originating from the plant material associatedwith the chlorophyll transition.
 39. A carrier comprising software that,when loaded in a computer, calculates a characteristic chlorophyllfluorescence image that is a measure for the quantum efficiency of thephotosynthetic activity of the photosynthetic system of the plantmaterial from chlorophyll fluorescence images Ffast and Fslow, whereinFfast is obtained by scanning with a high frequency a beam ofelectromagnetic radiation comprising one or more such wavelengths thatat least a part of the chlorophyll present is excitated by at least apart of the radiation, and having such a shape that only a small part ofthe plant material is irradiated, over the plant material and measuringthe fluorescence radiation originating from the plant materialassociated with the chlorophyll transition and Fslow is obtained byscanning with a low frequency a beam of electromagnetic radiationcomprising one or more such wavelengths that at least a part of thechlorophyll present is excitated by at least a part of the radiation,and having such a shape that only a small part of the plant material isirradiated, over the plant material and measuring the fluorescenceradiation originating from the plant material associated with thechlorophyll transition.